The present invention relates to magnetic disk drive data storage devices, and more particularly to the magnets used in rotary actuators employed in such devices to selectively position magnetic transducing heads relative to rotating magnetic disks.
Magnetic disk drives generally include either linear actuators or rotary actuators to position magnetic reading and recording heads with respect to rotatable magnetic disks. Such heads typically are moved approximately radially of the disks to seek and follow a selected one of multiple tracks of data stored on the disk.
Rotary actuators typically include a shaft mounted rotatably on a deck or stationary housing portion of the drive, along with a plurality of stacked transducer support arms connected to the shaft for movement with (or with respect to) the shaft. A magnetic transducing head is mounted to the end of each support arm remote from the shaft, and thus travels in an arcuate path as the arms pivot. The means for controllably pivoting the head arm assembly usually is an electric motor including a voice coil and permanent magnets. Typically the voice coil is a flat loop perpendicular to the shaft pivot axis, connected to the shaft. Permanent magnets, integral with disk drive housing, are positioned in spaced apart relation to the voice coil on opposite sides of the coil. When electrical current is carried through the voice coil, a magnetic field is generated which interacts with the magnetic field of the permanent magnet to provide the force which moves the voice coil and thus rotates the shaft.
There are certain disadvantages inherent in this arrangement. Only the radially outward portion of this voice coil has a desirably long moment arm, i.e. the distance from the pivot axis. The radially inward portions of the coil have shorter effective moment arms, and thus provide correspondingly reduced rotational force on the shaft and head arm assembly. Accordingly, U.S. patent application Ser. No. 451,200 filed Dec. 15, 1989 and assigned to the assignee of the present application, features a thin, arcuate voice coil curved about the pivot axis of the rotary actuator. The coil, formed of a single winding of aluminum wire, is integral with the head arm assembly. A pair of permanent magnets and the voice coil are curved about the pivot axis and spaced apart radially from one another to define a narrow, arcuate air gap. The radial dimension of the voice coil is its thickness, which is quite small compared to its length and width, accordingly, substantially all of the coil has the same moment arm or lever arm about the pivot axis, for more consistent and reliable performance of the rotary actuator.
The performance of the rotary actuator is a function of the permanent magnet or magnets as well as the voice coil. The typical magnet arrangement involves two magnets side by side. The magnets are uniform in their height, in the direction parallel to the pivot axis, and in their thickness in the radial direction. The width of the magnets is arcuate, such that the magnets form abutting segments of a circular cylinder.
For maximum efficiency in reciprocating the head arm assembly, the magnetic flux paths should be radial. Radial flux paths are perpendicular to the arcuate path of the coil, so that the applied force, perpendicular to magnetic flux, is tangential at each radius, i.e. arcuate over multiple radii. The magnets conventionally utilized in actuators are formed by scintering, however, which results in linear and parallel flux paths. Parallel flux paths deviate from the desired radial flux paths with increasing severity as the arcuate width of the magnet increases and the radial distance from the pivot axis decreases. Accordingly, the scintered magnet gives rise to strict physical design limitations, and an uneven distribution of force along the arcuate path of the voice coil.
Recently it has been found that arcuate magnets, when forged rather than scintered, can be formed with the desired radial magnetic flux paths. Forging, however, tends to increase the density of magnetic material in the center of each magnet relative to the peripheral regions. The result is an uneven distribution of magnetic force, in spite of the desired radial flux path orientation.
Therefore, it is an object of the present invention to provide a rotary actuator in which the permanent magnet arrangement provides an even distribution of forces along substantially all of an arcuate path of travel of a rotary actuator voice coil.
Another object of the invention is to provide a permanent magnet for a rotary actuator, shaped in a manner to compensate for gradients in the density of magnetic material from the edges of the magnet to its center.
Yet another object is to provide a rotary actuator for magnetic disk drives, in which the interaction of respective magnetic fields of a permanent magnet arrangement and an arcuate voice coil is characterized by a substantially uniform distribution force along the entire range of arcuate travel for the voice coil.